SodiumStar 20/2 High Power cw Tunable Guide Star Laser

Laser Guide Star

Adaptive Optics Facilities

LIDAR

Atmospheric Monitoring

Laser Cooling

High Power cw Tunable Guide Star LaserSodiumStar 20/2

Existing and upcoming next generation

optical telescopes require highly reliable

589 nm high-power laser sources with

excellent beam quality – so-called Guide

Star Lasers – for the implementation of

adaptive optics. Under contract from the

European Southern Observatory (ESO) and

the W.M. Keck Observatory, industrial part-

ners TOPTICA Photonics and MPB Com-

munications have jointly developed, pro-

duced and field-tested such a Guide Star

Laser for direct integration into the ESO VLT

telescope structure and established the

new standard for this emerging field.

Operational Principle

A master oscillator signal from a linearly po-

larized 1178 nm cw diode laser is stabilized

to a high resolution (10 MHz) wavelength

meter. Its output is efficiently amplified in a

polarization-maintaining (PM) Raman fiber

amplifier (RFA) based on ESO’s patented

technology. The spectrally narrow output

is mode-matched to an efficient resonant

frequency doubling cavity delivering more

than 20 W of optical output at 589 nm.

Modular Design and Highest Availabili-

ty for Astronomical Observation

Utmost care was taken to design and test

the laser unit for remote operation in the

harsh environments encountered on the top

of a mountain, driving thermal, vibrational

and even earthquake stability to the limits of

current technology. Total weight of 700 kg,

warm-up times of less than 30 minutes,

power consumption of about 700 W (@ BOL),

integrated optical sideband generation

(D2b repumping) and a novel remote pum-

ping option along with many other features

provide next generation telescope designers

with unprecedented degrees of freedom.

The SodiumStar represents a paradigm shift

in operational simplicity. This eliminates the

need for labor-intensive daily alignment tu-

ne-ups and lengthy system warm-up times.

The complete laser unit consists of line-re-

placeable units (LRUs) which can be swap-

ped on-site for service and repair. The main-

tenance is reduced to minimize downtime

and manpower requirements at the obser-

vatories. For future needs, the design offers

the possibility of scaling the output power

to even higher levels while preserving the

excellent beam quality.

Direct Integration into the Telescope

Structure

In contrast to the currently used dye or

solid-state laser systems which rely on a

stationary laser clean room at the observa-

tory facility, the SodiumStar can be directly

integrated into the telescope structure

with its gravity-varying condition. As part

of ESO‘s 4LGSF, four of these lasers

are remotely operated via an Ethernet

connection.

The distributed feedback based seed laser

technology guarantees:

· intrinsic single-mode operation

· precise wavelength stabilization

· fast wavelength toggling for measuring

the Rayleigh scattering background.

The high-power Raman fiber amplification

has its known strong points:

· surface-to-volume ratio of fiber lasers

allows favorable power scalability

· no need for optical alignment due to

all-fiber approach

· absence of linewidth broadening during

amplification.

Schematic of Laser Unit architecture compri-sing a semiconductor master oscillator serving as seed source for a PM Raman fiber ampli-fier which is pumped by a polarized fiber laser. The emission wavelength is stabilized via a highly accurate wavelength meter. Subsequent to amplification, the high-power laser radiation is converted to the yellow spectral region within the second-harmonic generation mo-dule (SHGM). Frequency sidebands for optical repumping are generated via current modulation of the seed laser.

Semiconductor Master

Oscillator

Wavelength Measurement

Fiber LaserPump Module

Second Harmonic

Generation

RFA

The frequency doubling technique employs

a newly-developed doubly-resonant SHG:

· highly efficient and robust

· fast control electronics

· based on state-of-the-art optomechanics

· active beam control options.

The system architecture avoids wave-

front-critical optical modulators such as

EOMs or AOMs and guarantees excellent

beam quality.

Remote Pumping: The inclusion of the

control electronics in the SHG module, in

combination with the flexibility of the RFA

/ fiber pump laser architecture, makes

“remote pumping” possible. This novel

technique allows spatial separation bet-

ween the compact Laser Head and the

Electronics Cabinet of 32 m and therefo-

re enables highly flexible telescope inte-

gration. The compact Laser Head can be

located for example in close proximity to

the laser launch telescope while the Elec-

tronics Cabinet containing the pump di-

odes, drive electronics and power supplies

(heat sources), can be installed in a more

convenient location well separated from the

launch telescope.

The engineering of a deployable laser

system based on ESO‘s original concept

has resulted in a ruggedized, compact and

modular solution, with focus on reliability

and serviceability. Options of support for

integration on-site, periodic inspections for

preventive maintenance as well as full ser-

vice support are offered.

Ready for IntegrationFully Engineered

SodiumStar 20/2 laser head with RFA (left) and SHGM (right)

on a water-cooled cold plate.

Schematic of the second harmonic generation module (SHGM).

SodiumStar 20/2 electronics cabinet.

Interior view of SodiumStar 20/2 electronics cabinet.Remote pumping option: The laser head can be separated up

to 32 meters from the electronics cabinet.

3 meters

Fiber Laser Power Supply

Fiber Laser Pump Module

Wavelength Measurement Module

Programmable Logic Controller Module

Power Entry Module

Main / Seed Laser Module

Hydraulic Module

27 meters

Emitted spectrum of the laser recorded via a scanning FPI with

sidebands for repumping (Na D2b).

Emission wavelength during climate chamber test ±40 MHz spec,

ambient temperature over 80 hours.

Shack-Hartmann wavefront measurement of the output beam

with rms wavefront error of 23 nm.

Fast detuning of emission frequency by 5 GHz within 2.5 s for

measuring the Rayleigh scattering background.

Long-term power measurement showing a mean value of 22 W with

peak-to-peak variation of 2.1 %.

Pointing and lateral shift of the laser output beam during a tilting test

of the laser head carried out at full operational power level of 22 W of

output power at 589 nm.

Specifications and Key Features

Key Features

· Robust and fully engineered system

· Direct integration into telescope

structure

· Gravity invariant operation

· All polarization-maintaining fiber

approach

· Excellent polarization properties

· High wall-plug efficiency

· Minimized cooling requirements

· State-of-the-art optomechanics

· Active beam control implemented

· Remote pumping scheme (optional)

TOPTICA Projects GmbH and MPB Communications, Inc. are licensees of the ESO Fiber Raman Amplifier Technology (EFRA) developed and transferred by ESO (www.eso.org).

Optical Specifications

Laser output power ≥ 20 W

Laser wavelength* 589.159 nm

Wavelength tunability ± 6 pm

Frequency stability ± 40 MHz

Laser linewidth ~ 5 MHz

Power stability peak-to-peak < 10 %

Intensity noise rms < 3 %

Optical beam diameter 4σ 3 ± 0.1 mm

Beam ellipticity < 5 %

Beam quality (rms wavefront error) < 50 nm

Beam pointing rms < 160 µrad

Beam shift rms < 100 µm

Polarization extinction ratio > 100:1

Linear polarization stability < 2°

Repumper amplitude 0 .. 15 %

Cooling System Specifications

Coolant temperature 15 °C

Temperature stability ± 0.75 K

Coolant flow rate ~ 5l/min

General Specifications

Electrical power consumption < 1 kW

Operating temperature range -10 .. 25 °C

Operating altitude (above sea level) < 4300 m

Dimensions laser head (including insulation cover) 44 x 93 x 73 cm³ (H x W x D)

Weigth laser head 80 kg

Dimensions electronics cabinet 173 x 91 x 93 cm³ (H x W x D)

Weigth electronics cabinet 600 kg

*other wavelength / beam diameter on request

Mechanical Drawings

TOPTICA Projects GmbHLochhamer Schlag 19D-82166 Graefelfing/MunichGermanyPhone: +49 89 85837-500Fax: +49 89 85837-510info@toptica-projects.comwww.toptica.com

BR-SodiumStar 20/2-2018-06

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705

90 Laser Beam Exit Point

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1730

910 930

INVISIBLE LASER RADIATIONAVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION

CLASS 4 LASER PRODUCT(DIN EN 60825-1:2014)